1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery comprising a positive electrode active material capable of occluding/ejecting lithium ions, a negative electrode active material capable of occluding/ejecting lithium ions, and a non-aqueous type electrolyte, and a method of manufacturing the non-aqueous electrolyte battery.
2. Description of the Related Art
In recent years, as batteries used for portable electronic and communication equipment such as small-sized video cameras, portable telephones, and book-type personal computers, non-aqueous electrolyte secondary batteries constituted by using an alloy or a carbon material capable of occluding/ejecting lithium ions as a negative electrode active material and lithium-containing transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2) and lithium manganese oxide (LiMn2O4) as a positive electrode active material have been put to practical use as batteries with high capacity capable of charging and discharging although small in size and light in weight.
Among these lithium-containing transition metal oxides used as positive electrode active materials in the manufacture of the above-described non-aqueous electrolyte secondary batteries, the lithium nickel oxide (LiNiO2).is known to have high capacity, but has rather poor stability and large overvoltage. Accordingly, the lithium nickel oxide is deemed inferior to lithium cobalt oxide. On the other hand, while lithium manganese oxide (LiMn2O4) is rich in resources and relatively inexpensive, it is not practical to use because it has a low energy density and manganese itself is soluble at high temperature. Accordingly, manganese oxide is considered inferior to lithium cobalt oxide such that lithium cobalt oxide (LiCoO2) is mostly used as lithium-containing transition metal oxide in the manufacturing industry.
However, since lithium cobalt oxide (LiCoO2) is 4V vs Li/Li+ or more, when lithium cobalt oxide is used as the positive electrode active material in a non-aqueous electrolyte secondary battery, it is dissolved in the positive electrode during each charge/discharge repeat cycle, causing the positive electrode to deteriorate, thereby affecting the capacity characteristics and load characteristics of the battery after the charge/discharge cycle is lowered. In Japanese Patent Laid-Open Publication Nos. H4(1992)-319260, 2000-123834, 2001-319652, 2002-198051, 2001-273896, 2001-068167, and 2004-047437, lithium-containing cobalt composite oxides represented by the general expression ┌LiCo1-XMXO2┘ in which different elements M such as V, Cr, Fe, Mn, Ni, Al, Ti and Zr are added during the synthetical preparation of the lithium cobalt oxide, have been proposed.
The dissolution of cobalt in the electrolyte in the lithium-containing cobalt composite oxides (represented by the general expression ┌LiCo1-XMXO2┘) proposed in the above-mentioned patent references, is of lesser magnitude than that which occurs in lithium cobalt oxide (LiCoO2). Accordingly, the load performance and charge/discharge cycle performance characteristics of the battery are improved.
However, in the lithium-containing cobalt composite oxides to which various elements were proposed to be added in the above patent references, since the different elements involved do not influence battery reaction (charge/discharge reaction), the battery capacity in effect diminishes as the amount of such elements added is increased, which in turn causes a reduction in charge/discharge efficiency. Further, since the addition of various elements lowers crystallizability, thermal stability of the battery likewise diminishes, leading to lower load performance. Additionally, there are still many possibilities of improving the charging/discharging performance ability of the battery.